What R-value do I need for my attic, walls and floors?

The number is the assembly's total R, not one product's rating. Subtract the R of sheathing, glazing and finishes, then divide the remainder by the material's R per inch to get the thickness you need in each space.

Does doubling the insulation thickness double the R?

No. R-value per inch generally decreases as thickness increases, so a doubled thickness buys less than double the R. Foil-faced polyisocyanurate is the documented exception whose R per inch increases with thickness.

How do US R-values compare with metric RSI?

US customary R-values are approximately 5.68 times as large as metric RSI values. Mixing the two systems without converting is the most common failure in the thickness procedure.

How does U-factor relate to the R-value I specified?

The U-factor of an assembly is the reciprocal relationship to its R-value, so take the reciprocal of U to obtain the equivalent R. The two numbers describe the same assembly from opposite ends.

5.68×US R vs metric RSI

R-5.5/inFoil-faced polyiso, aged 5–10 years

R-21Rated R visible on a faced fiberglass batt

5.68Conversion factor

Attic, walls, floors

Bello Haus — insulation R-values | bellohaus.net

R-value measures the thermal resistance of a two-dimensional barrier such as a layer of insulation, a window or a complete wall. This page gives the measured R per inch of common materials, the procedure that turns a target R into a thickness, and a worked example of how R-values add across layers.

bellohaus.net
Thermal resistance reference · R-values, layers and U-factor
R-values, R per inch, and how insulation layers add up

Attic · Walls · Floors

R-value reference index

5.68Conversion factorUS customary R-values are approximately 5.68 times as large as metric RSI values.

R-6.5Top of closed-cell spray foam rangeClosed-cell polyurethane spray foam measures R-5.5 to R-6.5 per inch.

R-1.6Low end of Thinsulate per inchThinsulate clothing insulation measures R-1.6 to R-2.9 per inch.

R-value per inch by material

Measured R-value per inch for the main insulation materials
Material	R per inch (US)	Metric RSI per metre	Note
Closed-cell polyurethane spray foam	R-5.5 – R-6.5	RSI 38 – 45	Top of the measured range
Polyisocyanurate spray foam	R-4.3 – R-8.3	—	Widest band of the spray products
Phenolic spray foam	R-4.8 – R-7	—	Overlaps polyiso spray at both ends
Foil-faced polyisocyanurate rigid panel, aged 5–10 years	R-5.5	—	Exception: R per inch rises with thickness
Thinsulate clothing insulation	R-1.6 – R-2.9	—	Low-density textile insulation
Faced fiberglass batt	rated R-21 on facing	—	Rated value printed on the facing
US R vs metric RSI	× 5.68	1	Conversion factor between the two systems

Measured R-value per inch for the main insulation materials

What R-value are you actually specifying?

You specify the total thermal resistance of an assembly — the barrier's R, not a material's R in isolation.

R-value is the measure of thermal resistance of a two-dimensional barrier such as a layer of insulation, a window or a complete wall. When a target is quoted for an attic or a wall, it is the R of the whole assembly, and the material choice is only the means of reaching it.

Because R-values are additive for multiple layers of materials placed in series, a target can be met by one thick layer or by several thinner layers whose R-values sum to the same total. The comparison that matters is therefore R per inch, which tells you how much thickness each material needs to buy the same resistance.

Read the target as the assembly's total R, not one product's rating.
Convert the target into required R per inch of the chosen material.
Check that the resulting thickness fits the cavity or the roof build-up.

Terms used on this page

R-value: The measure of thermal resistance of a two-dimensional barrier such as a layer of insulation, a window or a complete wall. Higher R means greater resistance to heat flow.
R per inch: R-value divided by the material's thickness in inches. It is the figure used to convert a target R into a required thickness.
RSI: The metric expression of thermal resistance. US customary R-values are approximately 5.68 times as large as metric RSI values.
U-factor: The reciprocal relationship to an assembly's R-value. It expresses the same assembly as conductance rather than resistance.
Series addition: The rule that R-values are additive for multiple layers of materials placed in series, so the assembly R is the sum of the layer R-values.
Aged R-value: The R-value measured after years in service rather than at installation; the foil-faced polyisocyanurate figure of R-5.5 per inch is an aged 5–10 year measurement.

Common questions

What R-value do I need for my attic, walls and floors?: The number is the assembly's total R, not one product's rating. Subtract the R of sheathing, glazing and finishes, then divide the remainder by the material's R per inch to get the thickness you need in each space.
Does doubling the insulation thickness double the R?: No. R-value per inch generally decreases as thickness increases, so a doubled thickness buys less than double the R. Foil-faced polyisocyanurate is the documented exception whose R per inch increases with thickness.
How do US R-values compare with metric RSI?: US customary R-values are approximately 5.68 times as large as metric RSI values. Mixing the two systems without converting is the most common failure in the thickness procedure.
How does U-factor relate to the R-value I specified?: The U-factor of an assembly is the reciprocal relationship to its R-value, so take the reciprocal of U to obtain the equivalent R. The two numbers describe the same assembly from opposite ends.

How this record was assembled

Every figure on this page comes from the measured table and the stated rules of R-value arithmetic: series addition, the 5.68 US-to-RSI factor, and the general decrease of R per inch with thickness. Where a value is aged or rated rather than measured per inch, the page says so in the row or the note.

Sources

Figures are taken from the measured R-value table and the stated relationships between R-value, R per inch and U-factor.